Gas turbines extract energy from a hot gas stream received from a combustor and the energy is used to power a fan, propeller, compressor or generator.
Each turbine includes an assembly of bladed discs mounted on and rotatable about a turbine bearing support structure and enclosed within a turbine casing.
The turbine bearing support structure typically contains internal service pipes for oil feed, scavenge and venting of bearing chambers, ducting high pressure air from the compressor and to provide air for sealing or cooling. All of these internal service pipes need to be connected to external pipework (i.e. pipework outside the turbine casing). The connections between the internal and external pipes are typically provided within the turbine casing which exposes the connections to the elevated temperatures and pressures within the turbine.
A prior art fastener assembly for connecting internal and external pipes in a bearing support structure is shown in FIG. 1.
An internal pipe 1 of the turbine bearing support is provided with a threaded adapter 2. The adapter 2 has a spherical sealing surface 3 provided with an anti-wear coating. A sleeve has a laterally extending portion 4 which is bolted to the casing 5 and an axially extending portion 6 which carries a sealing surface 7 which abuts the spherical sealing surface 3 on the adapter 2. The adapter 2 includes anti-rotation dog members 24 which are engaged in a slot in the axially extending portion 6 of the sleeve to react against the applied torque loads and prevent over-torquing of the pipe 1.
An external pipe of the turbine bearing support is provided with a ferrule 8 having a flange 9. The flange 9 is secured against the adapter 2 using a threaded nut 10, the threads of the threaded nut 10 mating with the threaded adapter 2. The components are locked into place using a positive locking device such as a locking wire or anti-rotation bracket.
There are a number of problems with this prior art fastener assembly.
Firstly, any axial or radial movement of the pipes resulting from, for example, build misalignment or tolerance build-up, thermal expansion or engine running conditions, may result in fretting or uneven wear at the sealing surfaces causing the seals to fail. This is particularly problematic with the spherical sealing surfaces of the prior art fastener assembly design which are difficult to manufacture consistently.
Whilst anti-wear/anti-fret coatings are provided on the sealing surfaces of the adapter 3 and the sleeve 7, the cost of these coatings is significant and they are often unsuitable for use at the elevated temperatures associated with gas turbines.
Furthermore, extraction of the prior art assembly for replacement of worn parts can be tricky as a result of the interference at the spherical sealing surfaces.